Fill Brush with Radial Gradient not working with Project Paint
I think that actually we need to think in regards to how to arrive at the radial gradient dots on the target image. The gradient fill brush is going to fill the whole image, and works from clicking somewhere in the image and pulling away from center to create a radial gradient fill.

My preferred way to do this is to use a Blend texture set to radial, and in the brush settings I set it to stroke type 'Anchored' and change 'Tiled' mapping to 'View Plane'. Define your gradient there in the place in the texture panel marked 'color' and then set your stops to the colors of your choice. If you already have a gradient worked out, hover over it and press ctrl-C to copy it, and hover over this color ramp and press ctrl-V to paste it in. Click in the image area on your mesh, and the dragging away you should get a preview of the colors and size before release.